1. Field of the Invention
This invention relates to a radiographic apparatus and a radiographic method for effecting phototaking by the use of radiation typified by X rays.
2. Related Background Art
Generally radiographic apparatuses are used in the fields of radiography for medical treatment, non-destructive radiography for industry, etc. Their form of use will hereinafter be described with reference to FIG. 1 of the accompanying drawings. When radiation radiated from a radiation source 1 is applied to an object 2, the radiation is intensity-modulated and scattered in conformity with the structure of the object by the mutual action (absorption, scattering, etc.) between the radiation and the object, and has its scattered rays removed by a grid 4, whereafter it arrives at a fluorescent material 5 as a radiation image. Generally, as the fluorescent material, use is made of an intensifying screen comprising CaWO4 or Gd202S:Tb applied to a back-up material or fluorescent material crystal such as CsI. The fluorescent material has a characteristic of emitting fluorescence of intensity proportional to the amount of applied radiation and therefore, the aforementioned radiation image is converted into a visible optical image in the fluorescent material 5. Image receiving means 6 is means for producing an image conforming to the quantity of received light, and the visible optical image produced in the fluorescent material is made into an image conforming to the quantity of light by the image receiving means 6. Generally in a radiographic apparatus, the image receiving means is film and a radiation image is recorded on the film as a latent image providing photographic density substantially proportional to the logarithm of the quantity of fluorescence, and is presented as a visible image after the developing process and is used for diagnosis or examination or the like.
Use has also begun to be made of a computed radiography apparatus using as a fluorescent material an imaging plate (hereinafter referred to as an IP, having applied thereto BaFBr:Eu fluorescent material and BaF:Eu fluorescent material, which are accelerated fluorescent materials. When secondary excitation is effected on the IP primarily excited by the application of radiation, by the use of visible light such as a red laser, emitted light called accelerated fluorescence is created. The computed radiography apparatus is an apparatus for detecting this emitted light by an optical sensor such as a photomultiplier to thereby phototake a radiation image.
Further, there has recently been developed a technique using as image receiving means a photoelectric conversion device comprising pixels comprising minute photoelectric conversion elements, switching elements or the like arranged in the form of a grid to obtain a digital image. There has also been developed a technique using a converting material for converting radiation into electrons, for example, amorphous selenium, CdTe or an ion chamber, and a detecting device for detecting electrons to obtain a digital image. As examples of the advantages of utilizing a radiation detecting device, the following items may be mentioned. First, an image can be directly obtained as digital data and therefore, image processing becomes easy and the correction of inappropriate phototaking conditions, the image emphasis of a concerned area, etc., easily become possible. Also, by using image communication means such as facsimile apparatus, it is possible for a medical specialist in a large hospital to effect a diagnosis to a patient in a remote place where there is no medical specialist. Also, if image digital data are stored in a magneto-optical disc or the like, the required preservation space can be remarkably decreased as compared with a case where film is stored. Also, images in the past can be easily searched for and therefore, it becomes possible to present a reference image easily as compared with a case where film is searched for.
A grid will now be described briefly with reference to FIG. 1 of the accompanying drawings. A grid 4 is generally a plate in which a lamination comprising leaden plates and aluminum plates alternately laminated has been thinly cut out, and acts to bring the leaden plates arranged substantially in parallel with one another into coincidence with the direction of travel of primary radiation to thereby eliminate unnecessary scattered radiation created from an object 2, and improve the contrast of a radiation image. The radiation image of the grid is superposed on the radiation image of the object and is recorded on image receiving means 6, and the spatial frequency of the grid is suitably selected, so that the image of the grid will not hinder to diagnosis. Generally 3 to 6 lines/mm are selected as the spatial frequency of the grid, and the more is reduced the spatial frequency of the grid, the clearer the image of the grid looks, but doctors are considered to effect diagnoses without being misled by the image of the grid by training.
However, a conventional art radiographic apparatus using the aforedescribed photoelectric conversion device suffers from a problem which will be described below.
The mesh of the grid is modulated and phototaken by a radiation detecting device such as a photoelectric conversion device for effecting discrete sampling. According to the sampling theorem, discrete sampling means typified by the photoelectric conversion device cannot reproduce an input image of Nyquist frequency or higher. Therefore, if the image of the grid has a spatial frequency higher than the Nyquist frequency of the photoelectric conversion device, the image of the grid is spatial-frequency-modulated and intensity-modulated and is observed as Moire stripes. The output image at this time becomes like a striped pattern of a spatial frequency greater the original grid frequency is superposed, for example, on the radiation image of an object. On the other hand, even when the discrete sampling means is not used, there is a basic requirement for removing the image of the grid superposed on the image of the object.